In a tubing and a casing used in digging an oil well, a threaded joint is generally used. Usually, a depth of an oil well is 2000 m to 3000 m. However, in a deep oil well such as a marine oil field in recent years, a depth of an oil well reaches 8000 m to 10000 m. An axial tensile force due to a weight of an oil well pipe and a joint themselves and a pressured which is complexed of an internal and external pressure, and heat in the ground exert on a threaded joint for fastening these oil well pipes under the use environment. Therefore, a threaded joint used in an oil well pipe is required to retain gas tightness without damage even under such the environment. In addition, at work of falling a tubing and a casing into an oil well, a once fastened joint is loosened due to a variety of troubles, they are once pulled out from an oil well, and the joint is fastened again, which is fallen.
API (American Petroleum Institute) requires that, even when fastening (makeup) and loosening (breakout) are performed ten times in a tubing joint, or three times in a casing joint, seizure called galling does not occur, and gas tightness is retained. In a threaded joint, usually, a male thread is formed at an end part of an oil well pipe, and a female thread is formed on an internal surface of a threaded joint member. And, by fitting a unthreaded metal contact part formed at a tip of a male thread and a unthreaded metal contact part formed at a base of a female thread, and fastening them, whereby, unthreaded metal contact parts area butted, and a metal seal part is formed. Upon fastening, galling resistance and gas tightness are improved by coating a viscous liquid lubricant containing a heavy metal powder, called “compound grease”. In addition, a threaded part and a unthreaded metal contact part of a threaded joint are subjected to surface treatment for the purpose of improving retainability of a compound grease and improving slidability.
However, since a high surface pressure exceeding a yield point of a threaded joint material is acted particularly on a unthreaded metal contact part of a threaded joint, galling easily occurs. Previously, various threaded joints in which galling resistance at such the site is improved have been proposed. For example, JP-A No. 61-79797 discloses a threaded joint in which a threaded part is plated with zinc (Zn) or tin (Sn), and a metal sealing part (unthreaded metal contact part) is plated with gold (Au) or platinum (Pt). JP-B No. 3-78517 discloses a pipe joint in which a film of a synthetic resin with molybdenum disulfide (MoS2) having a particle diameter of 10 μm or smaller dispersed therein at a ratio of 20 to 90% is formed. In addition, JP-A No. 8-103724 discloses a method of treating a surface of a steel pipe joint by forming a resin film containing molybdenum disulfide on a film layer chemically treated with manganese phosphate. Further, JP-A No. 8-105582 discloses a method of treating a surface of a pipe joint in which a nitrification-treated layer is a first layer, an iron-plated layer or an iron alloy-plated layer is a second layer, and a third layer of a resin film containing molybdenum disulfide is formed thereon.
All of threaded joints disclosed in the aforementioned respective patent references are on presume that a compound grease is used. This grease contains a heavy metal powder such as zinc, lead and copper, and since a coated grease is washed out at connecting of threads, and a coated grease is overflown on an outer surface at fastening in some cases, it is thought that this adversely affects on the environment, particularly, marine organisms. In addition, work of coating a compound grease has not only the worse working environment, but also feared harmfulness on a human body. Therefore, development of a threaded joint without using a compound grease is demanded.
However, it is difficult for the aforementioned previous techniques to ensure performance required for the aforementioned threaded joints. For example, it goes without saying that techniques disclosed in JP-A No. 5-117870, JP-A No. 6-10154, JP-A No. 5-149485 and JP-A No. 2-885593 in which coating of a compound grease stipulated in API standard BUL 5A2 is kept in mind at fastening of a threaded joint have a problem of an adverse effect on the earth environment and a human body.
In recent years, based on OSPAR Treaty (Oslo•Paris Treaty), discharge of substances harmful on the earth environment and substances having influence on a human body is being strictly regulated in development of a well of a natural gas and a petroleum. OSPAR Treaty was established by unifying Oslo Treaty and Paris Treaty regarding prevention of marine pollution in 1992, and it was settled that marine environmental protection in Northeast Atlantic Ocean is progressed. In this OSPAR Treaty, an idea of prevention principle is incorporated as general duty of Contracting States, and this is a first framework having a lawful restriction force. OSPAR Treaty was ratified by all countries contacting with Northeast Atlantic Ocean, and came into force in 1998. With progression of such the global strict regulation on the environment, a threaded joint which has no adverse effect on the environment and a human body at fastening of an oil well pipe even in work of digging a gas or oil well, and has excellent galling resistance is sought. The compound grease which has been previously used in a threaded joint upon fastening of a steel pipe for an oil well is being restricted in its use in some areas.
As a threaded joint using no compound grease, a threaded joint with a solid lubricant film formed on a surface is proposed. For example, JP-A No. 8-233163, JP-A No. 8-233164 and JP-A No. 9-72467 disclose a threaded joint in which a resin film obtained by dispersing molybdenum disulfide (MoS2) or tungsten disulfide (WS2) in a resin is formed. However, they are degraded under the high temperature environment to generate a sulfurous acid gas (sulfur dioxide: SO2), thus, it is feared that they have not a little effect on the environment.